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Feature

Today, Neptune has arrived at the same location in space where it
was discovered nearly 165 years ago. To commemorate the event,
NASA's Hubble Space Telescope has taken these "anniversary pictures"
of the blue-green giant planet.

Neptune is the most distant major planet in our solar system. German
astronomer Johann Galle discovered the planet on September 23, 1846.
At the time, the discovery doubled the size of the known solar
system. The planet is 2.8 billion miles (4.5 billion kilometers)
from the Sun, 30 times farther than Earth. Under the Sun's weak pull
at that distance, Neptune plods along in its huge orbit, slowly
completing one revolution approximately every 165 years.

These four Hubble images of Neptune were taken with the Wide Field
Camera 3 on June 25-26, during the planet's 16-hour rotation. The
snapshots were taken at roughly four-hour intervals, offering a full
view of the planet. The images reveal high-altitude clouds in the
northern and southern hemispheres. The clouds are composed of
methane ice crystals.

The giant planet experiences seasons just as Earth does, because it
is tilted 29 degrees, similar to Earth's 23-degree-tilt. Instead of
lasting a few months, each of Neptune's seasons continues for about
40 years.

The snapshots show that Neptune has more clouds than a few years ago,
when most of the clouds were in the southern hemisphere. These
Hubble views reveal that the cloud activity is shifting to the
northern hemisphere. It is early summer in the southern hemisphere
and winter in the northern hemisphere.

In the Hubble images, absorption of red light by methane in
Neptune's atmosphere gives the planet its distinctive aqua color.
The clouds are tinted pink because they are reflecting near-infrared
light.

A faint, dark band near the bottom of the southern hemisphere is
probably caused by a decrease in the hazes in the atmosphere that
scatter blue light. The band was imaged by NASA's Voyager 2
spacecraft in 1989, and may be tied to circumpolar circulation
created by high-velocity winds in that region.

This illustration is a composite of numerous separate Hubble WFC3 images. A color image composed of exposures made through three color filters shows the disk of Neptune, revealing clouds in its atmosphere. 48 separate images from a single filter were brightened to reveal the very faint moons. The white dots are Neptune's inner moons moving along their orbits during Hubble's observations. The solid green lines trace the full orbit of each moon. The spacing of the moon images follows the timing of each Hubble exposure. About 30 moons are known to orbit Neptune, most of which are too faint or orbit too far away to appear in these images. Illustration credit: NASA, ESA, and Z. Levay (STScI)› Larger image

Neptune has an intriguing history. It was Uranus that led
astronomers to Neptune. Uranus, the seventh planet from the Sun, is
Neptune's inner neighbor. British astronomer Sir William Herschel
and his sister Caroline found Uranus in 1781, 55 years before
Neptune was spotted. Shortly after the discovery, Herschel noticed
that the orbit of Uranus did not match the predictions of Newton's
theory of gravity. Studying Uranus in 1821, French astronomer Alexis
Bouvard speculated that another planet was tugging on the giant
planet, altering its motion.

Twenty years later, Urbain Le Verrier of France and John Couch Adams
of England, who were mathematicians and astronomers, independently
predicted the location of the mystery planet by measuring how the
gravity of a hypothetical unseen object could affect Uranus's path.
Le Verrier sent a note describing his predicted location of the new
planet to the German astronomer Johann Gottfried Galle at the Berlin
Observatory. Over the course of two nights in 1846, Galle found and
identified Neptune as a planet, less than a degree from Le Verrier's
predicted position. The discovery was hailed as a major success for
Newton's theory of gravity and the understanding of the universe.

Galle was not the first to see Neptune. In December 1612, while
observing Jupiter and its moons with his handmade telescope,
astronomer Galileo Galilei recorded Neptune in his notebook, but as
a star. More than a month later, in January 1613, he noted that the
"star" appeared to have moved relative to other stars. But Galileo
never identified Neptune as a planet, and apparently did not follow
up those observations, so he failed to be credited with the
discovery.

This video sequence compiles data from Hubble's observations of Neptune to show the blue-green planet rotating on its tilted axis. A day on Neptune is 16 hours long, and Hubble took images of the planet every four hours. Some of Neptune's moons are also shown. (No audio.)
Video Credit: NASA, ESA, and G. Bacon (STScI)
Science Credit: The Hubble Heritage Team (STScI/AURA) and A. Simon-Miller (NASA Goddard)

Neptune is not visible to the naked eye, but may be seen in
binoculars or a small telescope. It can be found in the
constellation Aquarius, close to the boundary with Capricorn.

Neptune-mass planets orbiting other stars may be common in our Milky
Way galaxy. NASA's Kepler mission, launched in 2009 to hunt for
Earth-size planets, is finding increasingly smaller extrasolar
planets, including many the size of Neptune.

Hubble is a project of international cooperation between NASA and
the European Space Agency. Goddard manages the telescope. The Space
Telescope Science Institute (STScI) conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy Inc. in Washington.